1. Field of the Invention
The present invention relates to a control apparatus for a fuel cell stack which includes a stacked body consisting of fuel cell units stacked together, and a pair of end plates which together sandwich the stacked body.
2. Description of the Related Art
In general, a solid polymer electrolyte fuel cell stack includes a plurality of fuel cell units, each of which includes a membrane electrode assembly, which is formed by sandwiching a polymer ion exchange membrane (i.e., a cation-exchange membrane) as an electrolyte membrane by an anode electrode and a cathode electrode, and a pair of separators which together hold the membrane electrode assembly. Such a solid polymer electrolyte fuel cell stack is practically used as a fuel cell stack having a predetermined number of fuel cell units being stacked together.
In such a fuel cell stack, a fuel gas (e.g., hydrogen gas) supplied to the anode electrode is ionized on a catalytic electrode, and moves to the cathode electrode through the electrolyte membrane which is moderately moistened. The electrons produced during this process are sent to an exterior circuit, and used as DC energy. Because the cathode electrode is provided with an oxidizing gas (e.g., air containing oxygen), water is generated at the cathode electrode through the reaction of the hydrogen ions, the electrons, and oxygen. This water is, hereinafter, referred to as “generated water” when appropriate. The amount of such water held in the fuel cell stack increases as the power generation in the fuel cell stack continues.
In addition, because the power generation reaction is accompanied by heat, the temperature of the fuel cell stack increases as the power generation in the fuel cell stack continues. A technique is known in the art in which a cooling medium is supplied to a fuel cell stack in order to maintain the temperature of the fuel cell stack within an appropriate range (see, for example, Japanese Unexamined Patent Application, First Publication No. Sho 64-27164).
In the above published Patent Application, a technique is disclosed in which an electrical heater is provided for heating the cooling medium as necessary, and when the temperature of the fuel cell stack is decreased during a slow power generation operation, the electrical heater is operated, and the cooling medium heated by the electrical heater is supplied to each of fuel cell units so that the entire fuel cell stack is warmed up.
In general, the fuel cell units disposed near the ends of the fuel cell stack tend to have more influence of the exterior temperature than in the case of the fuel cell units disposed in the middle portion of the fuel cell stack. Therefore, the variation of temperature in the fuel cell units cannot be reduced even when the heated cooling medium is supplied to the fuel cell stack, and the temperature of the fuel cell units disposed near the ends of the fuel cell stack tends to be lower than that of the fuel cell units disposed in the middle portion of the fuel cell stack when the power generation operation is stopped and supply of heat is stopped. Accordingly, even when substantially the same electrical power is generated in each of the fuel cell units and substantially the same amount of water is generated in each of the fuel cell units, relative humidity in the fuel cell units disposed near the ends of the fuel cell stack tends to be higher than that in the fuel cell units disposed in the middle portion of the fuel cell stack because the temperature of the fuel cell units disposed near the ends of the fuel cell stack tends to be lower than that of the fuel cell units disposed in the middle portion of the fuel cell stack; therefore, the amount of water held in the fuel cell units disposed near the ends of the fuel cell stack tends to be greater than that in the fuel cell units disposed in the middle portion of the fuel cell stack. As a result, in particular, the fuel cell units disposed near the ends of the fuel cell stack may have a problem in that water held in the fuel cell units freezes under low temperature conditions. If the water has frozen, the water (i.e., ice) inhibits the reaction gases (the fuel gas and oxidizing gas) from being sufficiently supplied to the anode electrode and the cathode electrode of the fuel cell units when the reaction gases are supplied to the fuel cell stack. As a result, the efficiency of power generation is decreased.